The present invention relates to methods of making aluminum alloys, and more particularly to a method of alloying aluminum with highly reactive metallic elements which tend to increase the oxidation rate of the melt, as for example, lithium.
Prior methods for melting aluminum alloys in preparation for ingot casting practices generally employ the addition of alloying elements to the aluminum melt in large melting furnaces. While minor alloying elements such as titanium-boron may be added at some point after the molten metal has left the melting furnace, this method is not employed for major alloying elements where concentrations of such elements must be carefully controlled. One of the major problems encountered with prior art alloying methods is that hydrogen is absorbed by the molten metal as a result of the reaction between the melt and the moisture in the air in the open furnaces. To minimize oxidation reactions, molten salt flux has been used on top of the molten metal. Yet even where the flux has been employed, large concentrations of hydrogen are found in the melt, and, unless removed, cause an unacceptable degree of porosity in the ingots. This "degassing", as the practice is commonly known in the art, is accomplished by bubbling a gas, usually containing chlorine, through the melt. A final degassing may be performed in a smaller holding furnace located closer to the casting station.
When lithium is used as an alloying element, it can only be added after all degassing has been completed. Lithium is added in either a solid or liquid state to the melting furnace or to the smaller holding furnace. In either case, it must be held under the surface of the melt until it dissolves.
Many difficulties have been encountered with lithium additions. Principally lithium has been found to significantly increase the oxidation rate of the melt. Lithium also contaminates the refractories in the furnace lining, making the furnace unsuitable for melting lithium-free alloys without a costly relining of the furnace. Special refractories and materials which are resistant to attack by lithium must be used throughout the melting and metal transfer stages. Moreover, the aluminum melt cannot be effectively degassed after the lithium is added without a resulting loss of much of the lithium. Thus because of the increased rate of oxidation after the addition of lithium, as well as the long holding time before all the metal finally solidifies in the ingot, significant loss of the costly lithium is very probable, and control of the desired concentration of lithium becomes critical. Another problem encountered concerns the considerable quantities of hydrogen which are absorbed by the melt after the lithium is introduced. Stirring the melt, after addition of the lithium, is performed to encourage uniformity of the composition, but in fact disrupts the protective cover of flux and results in further oxidation and hydrogen absorption. One proposed method for controlling this undesired oxidation involves covering the entire system (melting furnace, metal transfer troughs, holding furnace, and ingot casting station) and maintaining a dry, inert atmosphere over the molten metal. Unfortunately, implementation of this method would require redesign and reconstruction of the furnaces. Use of the method would also complicate control and monitoring of the casting procedures and equipment, and would be financially prohibitive.